Interfacial polymerization thin layer deposition

Interfdci l Composite Membra.nes, A method of making asymmetric membranes involving interfacial polymerization was developed in the 1960s. This technique was used to produce reverse osmosis membranes with dramatically improved salt rejections and water fluxes compared to those prepared by the Loeb-Sourirajan process (28). In the interfacial polymerization method, an aqueous solution of a reactive prepolymer, such as polyamine, is first deposited in the pores of a microporous support membrane, typically a polysulfone ultrafUtration membrane. The amine-loaded support is then immersed in a water-immiscible solvent solution containing a reactant, for example, a diacid chloride in hexane. The amine and acid chloride then react at the interface of the two solutions to form a densely cross-linked, extremely thin membrane layer. This preparation method is shown schematically in Figure 15. The first membrane made was based on polyethylenimine cross-linked with toluene-2,4-diisocyanate (28). The process was later refined at FilmTec Corporation (29,30) and at UOP (31) in the United States, and at Nitto (32) in Japan. 

The condensation polymerization process, employed recently by Skourlis et al. (1993) and Duvis et al. (1993), involves immersion of carbon fibers in a solution containing hexamethylenediamine and sodium carbonate. Dried carbon fibers are then immersed in a dipolychloride solution in carbon tetrachloride where the interfacial polycondensation reaction takes place. The result is that a thin layer of polyamide (nylon 6,6) coating is deposited on the continuous carbon fiber, whose thickness is controlled though by varying the diamine concentration. 

Interfacial polymerization provides another method for depositing a thin layer upon a porous support [47,49]. In this case polymerization occurs between the two reactive monomers at the interface of the two-immiscible solvents. Heat treatment is often applied to complete the interfacial polymerization, to cross-link water-soluble monomer or prepolymer. The advantage of this technique is that the reaction is inhibited by the passage of a limited supply of reactants through the already formed polymer layer, resulting in extremely thin film of thickness in the range of 1-2 nm. Such membranes are therefore referred to as thin film composites. TFCs of various poly(amides) are very popular in RO apphcations. 

Among these methods, interfacial polymerization developed by Cadotte and coworkers in the 1970s is currently the most widely used to prepare high performance reverse osmosis and nanofiltration membranes [45]. A thin selective layer is deposited on top of a porous substrate membrane by interfadal in-situ polycondensation. There... 

The solution coating technique was used in the preparation of the cellulose triacetate membrane discussed above. A solution of cellulose triacetate in chloroform was deposited on the porous support and the solvent was then evaporated leaving a thin film on the porous support. Thin film polymerization was used to prepare a polyfuran membrane barrier layer on polysulfone. In this case, the monomer furfuryl alcohol is polymerized in situ by adjustment of pH and temperature. This membrane proved to be highly susceptible to oxidizing agents and is of limited value. By far the most valuable technique in the formation of membrane barrier layers is interfacial polycondensation. In this method, a polymer is formed on the porous support surface at the interface of organic and aqueous phases by reaction of specific molecules dissolved in each phase. It is by this method that a number of polyamides and polyurea membrane barrier layers have been formed on polysulfone. Elements containing these membranes are available commercially. 

As already mentioned, TFC membranes are fabricated by in situ polymerization. This method, developed by Cadotte and coworkers of Filmtec in the 1970s, is currently most widely used to prepare high-performance RO and nanofiltration (NF) membranes [8]. A thin selective layer is deposited on top of a porous substrate membrane by interfacial in situ polycondensation. There are a number of modifications of this method primarily based on the choice of the monomers [13]. However, for the matter of simplicity, the polycondensation procedure is described by a pair of diamine and diacid chloride monomers. 

The next approach used the int acial polycondensation reaction to form a vay thin polymeric layer onto a substrate. Morgan first proposed this approach (Morgan, 1965), and then Scala et al. (1973) and Van Heuven (1976) actually applied this approach to obtain an RO membrane. But it was Cadotte who invented the high-performance membrane using the in situ interfacial condensation method (Cadotte, 1985). In his method, interfacial condensation reactions between polymeric polyamine and monomeric polyfimctional acid halides or isocyanates takes place on a substrate material to deposit a thin film barrier onto a substrate. Some of the composite membranes were succeeded in industrial fabrication by another method, which was designated as PA-300 or RC-100. 

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